The demand for poly(butylene terephthalate), hereinafter PBT, copolyetherester elastomers containing PBT hard segments, and poly(butylene isophthalate), hereinafter PBI, continues to grow.
There are numerous patents directed to processes for the continuous preparation of PBT from dimethyl terephthalate and 1,4-butanediol, hereinafter DMT and BDO respectively. Typically, these continuous processes employ a first transesterification stage wherein DMT and a substantial excess of BDO are contacted in the presence of a transesterification catalyst and reacted at around atmospheric pressure and at temperatures of 170.degree.-200.degree. C. During the transesterification reaction, methanol is formed as a by-product. In typical continuous processes, the bulk of the methanol thus formed is removed overhead through a fractionating column while any unreacted BDO is retained in the reaction mixture. The first transesterification stage is usually followed by two or more reaction stages operating at reduced pressure wherein the removal of methanol formed as a by-product of the transesterification reaction is completed and sufficient precondensation to form an oligomer (or prepolymer) suitable for feed to a continuous polycondensation reactor is achieved.
A recent disclosure of such a continuous process for preparing PBT from DMT and BDO is provided by U.S. Pat. No. 4,499,261 to Heinze et al. In the specific example of this reference, two transesterification stages and two precondensation stages are used prior to sending the resulting oligomer (or prepolymer) to the final polycondensation reactor. The first transesterification stage is operated at a pressure of 1.3 bar. The next three stages required to prepare the oligomer (or prepolymer) are operated at subatmospheric pressure. The total residence time of the reactants in the four stages prior to polycondensation is 240 minutes. A 30% molar excess of BDO relative to DMT is used. The loss of BDO resulting from the formation of tetrahydrofuran, hereinafter THF, is stated to be 5.1 moles per 100 moles of DMT fed.
While the above reference indicates that the loss of BDO to THF in the process therein is substantially less than encountered in earlier processes, a loss of 5.1 moles of BDO per 100 moles of DMT still represents a serious loss of an expensive raw material, especially when it is realized that the desired product is being manufactured at levels of tens of million pounds a year.
Beyond this yield loss, the process of the reference inherently represents a source of environmental contamination by virtue of the fact that several stages prior to the final polycondensation (i.e., precondensation stages) are operated at reduced pressure. This is a problem shared by prior continuous processes, along with batch processes, in general. Operation of the precondensation stages at reduced pressure on a commercial scale generally requires vacuum sources, such as, for example, steam jets or vacuum pumps. When a vacuum source is used, environmental contamination can occur because some of the volatile organics being removed in the precondensation stages cannot be fully condensed. The organic volatiles can then be emitted into the environment from the vacuum source. Accordingly, when vacuum operations are used in continuous reaction process, and also in batch processes, additional measures are necessary to protect the environment from the volatile organics that may be emitted by the vacuum source.
Thus, while the continuous preparation of PBT from BDO and DMT is well advanced as a result of numerous extensive investigations, there is still a need for improvement with regard to utilization of BDO and with regard to contamination of the environment with volatile organics. Likewise, the same type of improvements are desired in preparing PBI and copolymers based upon PBT or PBI.